Magnetic recording medium

ABSTRACT

The present invention provides a magnetic recording medium comprising a thin film magnetic layer of thickness in a range from 0.03 to 0.30 μm, and having excellent surface smoothness and superior electromagnetic conversion characteristics. The magnetic recording medium comprises a lower non-magnetic layer containing at least a non-magnetic powder and a binder resin on one surface of a non-magnetic support, an upper magnetic layer containing at least a ferromagnetic powder and a binder resin on the lower non-magnetic layer, and a back coat layer on the other surface of the non-magnetic support, wherein the thickness of the upper magnetic layer is within the range from 0.03 to 0.30 μm, the AFM surface roughness Ra of the upper magnetic layer is 6 nm or less, and the number of concavities with a depth of 30 nm or greater in the surface of the upper magnetic layer is 5 per 1 cm 2  of surface area or less.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a magnetic recording medium, andmore particularly to a magnetic recording medium having excellentsurface smoothness and superior electromagnetic conversioncharacteristics.

[0003] 2. Disclosure of the Related Art

[0004] A conventional magnetic recording medium comprises a magneticlayer on one side of a non-magnetic support and a back coat layer on theother side of the non-magnetic support to enhance the runningdurability.

[0005] In recent years, increases in the quantity of data being recordedhave resulted in demands for similar increases in the recording densityof magnetic recording media. In order to enable increases in recordingdensity, a recording wavelength continues to shorten, and the thicknessof the magnetic layer continues to reduce.

[0006] As the thickness of the magnetic layer is reduced, any surfaceroughness of the support is reflected in the surface of the magneticlayer, causing a loss in the smoothness of the magnetic layer surfaceand a deterioration in the electromagnetic conversion characteristics.As a result, a non-magnetic layer is provided on the support surface,for example, as an undercoat layer, and the magnetic layer is thenprovided on this non-magnetic layer.

[0007] As increasingly shorter recording wavelengths are being used, itis desirable to provide a flatter and improved mirror surface to themagnetic layer surface in view of spacing loss.

[0008] In view of the method for producing a magnetic recording medium,the magnetic layer is subjected to calendering to improve the smoothnessof the magnetic layer surface. Calendering of the magnetic layerfollowing formation of the back coat layer on the other side of thesupport has previously been reported.

[0009] For example, paragraph [0022] of Japanese Patent Laid-OpenPublication No. Hei 9-185822(1997) reports that by conductingcalendering with the magnetic layer and the back coat layer alreadyprovided on the support, the surface workability of the magnetic layerimproves markedly, resulting in improvements in other characteristicssuch as the electromagnetic conversion characteristics.

[0010] Japanese Patent Laid-Open Publication No. Hei 11-31322(1999)discloses a production process for a magnetic recording mediumcomprising a magnetic layer on one side of a support, which alreadycomprises a thin film formed by vacuum deposition on at least this side,and a back coat layer provided on the other side of the support, whereincalendering of the magnetic layer is conducted following formation ofthe back coat layer.

[0011] Japanese Patent Laid-Open Publication No. Sho 62-125537(1987)discloses a process in which calendering of the magnetic layer and theback coat layer are conducted simultaneously while the applied coatingsare cured by irradiation. However, a non-magnetic layer is not mentionedin this publication, and the thickness of the magnetic layer, at 3.0 to4.8 μm, is considerably high.

SUMMARY OF THE INVENTION

[0012] Accordingly, it is an objective of the present invention toprovide a magnetic recording medium capable of controlling the problemsof fluctuating factor in the film thickness of the magnetic layer andconcavity-convexity in the surface smoothness that are associated withreductions in the thickness of the magnetic layer to no more than 0.3μm, which represents the most suitable thickness value for shortwavelength recording, wherein the medium comprises a thin film magneticlayer of thickness in a range from 0.03 to 0.30 μm, and displaysexcellent surface smoothness and superior electromagnetic conversioncharacteristics.

[0013] In one aspect, the present invention provides a magneticrecording medium comprising a lower non-magnetic layer containing atleast a non-magnetic powder and a binder resin on one surface of anon-magnetic support, an upper magnetic layer containing at least aferromagnetic powder and a binder resin on the lower non-magnetic layer,and a back coat layer on the other surface of the non-magnetic support,wherein the thickness of the upper magnetic layer is within the rangefrom 0.03 to 0.30 μm, the AFM surface roughness Ra of the upper magneticlayer is 6 nm or less, and the number of concavities with a depth of 30nm or greater in the surface of the upper magnetic layer is 5 per 1 cm²of surface area or less.

[0014] Preferably, the average major axis length of the ferromagneticpowder is 0.1 μm or less.

[0015] Preferably, the magnetic recording medium is used in a recordingand reproducing system in which the minimum recording wavelength is 0.6μm or shorter.

[0016] Furthermore, another aspect of the present invention provides aproduction process for a magnetic recording medium comprising a lowernon-magnetic layer on one surface of a non-magnetic support, an uppermagnetic layer with a thickness of 0.03 to 0.30 μm on the lowernon-magnetic layer, and a back coat layer on the other surface of thenon-magnetic support, comprising a step A of forming the lowernon-magnetic layer by applying a non-magnetic layer coating containingat least a non-magnetic powder and a binder resin onto one surface ofthe non-magnetic support and subsequently drying and curing the coating,a step B of forming the upper magnetic layer by applying a magneticlayer coating containing at least a ferromagnetic powder and a binderresin onto the lower non-magnetic layer and subsequently drying thecoating, a step C of forming the back coat layer by applying a back coatlayer coating onto the other surface of the non-magnetic support andsubsequently drying the coating, and a step D of performing calenderingfollowing completion of both the step A and the step C.

[0017] Preferably, the calendering of step D is performed followingcompletion of both the step A and the step C but prior to the step B,and additional calendering of the step D is also performed followingcompletion of the step B.

[0018] Preferably, the average major axis length of the ferromagneticpowder used in the process is 0.1 μm or less.

[0019] Preferably, in the process, the AFM surface roughness Ra of theproduced upper magnetic layer is 6 nm or less, and the number ofconcavities with a depth of 30 nm or greater in the surface of the uppermagnetic layer is 5 per 1 cm² of surface area or less.

[0020] According to the present invention, there is provided a magneticrecording medium comprising a thin film magnetic layer with a thicknessof 0.03 to 0.30 μm that is ideally suited to short wavelength recordingin which the minimum recording wavelength is 0.6 μm or shorter, whichoffers excellent surface smoothness and superior electromagneticconversion characteristics. A magnetic recording medium of the presentinvention is particularly suitable as a linear recording tape for usewith computers.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Specific features of the present invention will now be describedin detail.

[0022] A magnetic recording medium in accordance with the presentinvention comprises a lower non-magnetic layer on one surface of anon-magnetic support, an upper magnetic layer with a thickness of 0.03to 0.30 μm on the lower non-magnetic layer, and a back coat layer on theother surface of the non-magnetic support. In the present invention, alubricant coating and various protective coatings for protecting themagnetic layer may be applied over the magnetic layer if necessary. Toimprove adhesion of the applied coating and the non-magnetic support andfor other purposes, a undercoat layer (adhesive layer) may be disposedon the surface of the non-magnetic support on which the magnetic layeris to be formed. The magnetic recording medium of the present inventionmay now be described with regard to each layer.

Lower Non-Magnetic Layer

[0023] The lower non-magnetic layer contains carbon black, non-magneticinorganic powders other than carbon black, and a binder resin.

[0024] Carbon black for use in the non-magnetic layer may be furnaceblack for rubbers, thermal black for rubbers, black for color, andacetylene black. Preferably, the carbon black has a specific surfacearea of 5 to 600 m²/g, a DBP oil absorbance of 30 to 400 ml/100 g, and aparticle size of 10 to 100 nm. Suitable carbon blacks are listed in“carbon black guide book” (ed., Carbon Black Association).

[0025] It is preferred that the carbon black contains minimal amounts ofwater-soluble sodium ions and water-soluble calcium ions: the amount ofthe water-soluble sodium ions is preferably 500 ppm or less, morepreferably 300 ppm or less while the amount of the water-soluble calciumions is preferably 300 ppm or less, more preferably 200 ppm or less.When contained in amounts greater than the specified range, thewater-soluble sodium ions or the water-soluble calcium ions may formsalts with organic acids (in particular, fatty acids) present in thecoating. Such salts may seep out to the surface of the coating, causingdrop-outs or an increase in the error rate.

[0026] To minimize the amounts of the water-soluble sodium ions and thewater-soluble calcium ions in the carbon black, the purity of water usedto terminate the reaction during the production of the carbon black orthe purity of water used in the granulation process may be increased.Production processes of carbon black are described in Japanese PatentLaid-Open Publication No. Hei 11-181323(1999), Japanese Patent Laid-OpenPublication No. Hei 10-46047(1998), and Japanese Patent Laid-OpenPublication No. Hei 8-12898(1996).

[0027] Various inorganic powders other than carbon black may be added tothe non-magnetic layer. Examples of the inorganic powders includeneedle-shaped non-magnetic iron oxide (a-Fe₂O₃), CaCO₃, titanium oxide,barium sulfate, and a-Al₂O₃. Preferably, the inorganic powder containsminimal amounts of water-soluble sodium ions and water-soluble calciumions: water-soluble sodium ions are preferably contained in an amount of70 ppm or less, more preferably 50 ppm or less. When contained inamounts greater than the specified range, the water-soluble sodium ionsmay form salts with organic acids (in particular, fatty acids) presentin the coating. Such salts may seep out to the surface of the coating,causing drop-outs or an increase in the error rate. To minimize theamounts of the water-soluble sodium ions and the water-soluble calciumions, the inorganic powders may be washed with water.

[0028] The ratio by mass of the carbon black to the inorganic powdersother than carbon black is preferably in the range of 100/0 to 5/95. Ifthe proportion of the carbon black is less than 5 parts by mass, thenthe surface electric resistance is adversary affected.

[0029] Aside from the above-described materials, the lower non-magneticlayer may contain binders such as thermoplastic resins, thermosetting orthermoreactive resins, and radiation-curable (electron beam- orUV-curable) resins. These binder resins are used in proper combinationsdepending on the characteristics of the magnetic recording medium andthe conditions for the process.

[0030] A preferred thermoplastic resin is one that has a softeningtemperature of 150° C. or below, an average molecular weight of 5000 to200000, and a degree of polymerization of approximately 50 to 2000. Apreferred thermosetting resin, thermoreactive resin, or aradiation-curable resin is one that has an average molecular weight of5000 to 200000 and a degree of polymerization of approximately 50 to2000 and can increase its molecular weight by undergoing condensation,addition, or other reaction processes when heated and/or irradiated withradiation (electron beam or UV) after being coated, dried, andcalendered.

[0031] Of these binder resins, particularly preferred are combinationsof nitrocellulose and polyurethane resins and combinations of vinylchloride type copolymers and polyurethane resins.

[0032] A preferred vinyl chloride type copolymer is one with the vinylchloride content of 60 to 95% by mass, in particular 60 to 90% by mass,and with the average degree of polymerization of approximately 100 to500.

[0033] Examples of such vinyl chloride type resins include vinylchloride-vinyl acetate-vinyl alcohol copolymers, vinylchloride-hydroxyalkyl(meth)acrylate copolymers, vinyl chloride-vinylacetate-maleic acid copolymers, vinyl chloride-vinyl acetate-vinylalcohol-maleic acid copolymers, vinyl chloride-vinylacetate-hydroxyalkyl(meth)acrylate copolymers, vinyl chloride-vinylacetate-hydroxyalkyl(meth)acrylate-maleic acid copolymers, vinylchloride-vinyl acetate-vinyl alcohol-glycidyl(meth)acrylate copolymers,vinyl chloride-hydroxyalkyl(meth)acrylate-glycidyl(meth)acrylatecoplymers, and vinyl chloride-hydroxyalkyl(meth)acrylate copolymers. Ofthese, copolymers of vinyl chloride and monomers having an epoxy(glycidyl) group are particularly preferred.

[0034] A preferred vinyl chloride type copolymer is one that containssulfate group (—OSO₃Y) and/or sulfo group (—SO₃Y), which are each apolar group and are referred to as S-containing polar group,hereinafter, to increase the dispersibility. While Y in the S-containingpolar groups may be any of H and alkali metals, particularly preferredS-containing polar groups are those in which Y is potassium, namely,—OSO₃K and —SO₃K. The vinyl chloride type copolymer may contain eitherone of the two S-containing polar groups or it may contain both of them,in which case the two polar groups may be contained at any proportions.

[0035] “Polyurethane resin” for use with the vinyl chloride type resinis a general term that encompasses all resins obtained through reactionsbetween a hydroxyl-containing resin, such as a polyester polyol and/or apolyether polyol, and a polyisocyanate-containing compound. Such resinshave an average molecular weight of approximately 5000 to 200000 and aQ-value (defined as mass average molecular weight/number averagemolecular weight) of approximately 1.5 to 4.

[0036] A preferred polyurethane resin may include a polar group on endsor side chains thereof. Polyurethane resins having a polar group withsulfur and/or phosphorus are particularly preferred.

[0037] Examples of the polar groups contained in the polyurethane resinare S-containing groups, such as —SO₃M, —OSO₃M, and —SR, P-containingpolar groups, such as —PO₃M, —PO₂M, —POM, —P═O(OM₁)(OM₂), and—OP═O(OM₁)(OM₂), —COOM, —OH, —NR₂, —N⁺R₃X⁻ (where M, M₁, and M₂ are eachindependently H, Li, Na, or K; R is H or a hydrocarbon; and X is halogenatom), epoxy group, and —CN. Preferably, the polyurethane resin usedincludes at least one of these polar groups, which is introduced intothe resin molecules through copolymerization or addition reaction. Thepolar group is preferably contained in the resin molecule in an amountof 0.01 to 3% by mass and may present either in the main chain of theresin molecules or in their branches.

[0038] Preferably, the polyurethane resin has a glass transitiontemperature Tg in the range of −20° C. to 80° C.

[0039] Using known techniques, such polyurethane resins can be obtainedby reacting, in the presence or in the absence of a solvent, a materialthat contains a compound having a particular polar group and/or a resinmaterial reacted with a compound having a particular polar group.

[0040] Aside from the vinyl chloride type copolymer and the polyurethaneresin, various known resins may be added to the non-magnetic layer in anamount of 20% by mass or less with respect to the amount of the entirebinder.

[0041] Examples of the thermoplastic resins other than the vinylchloride type copolymers and the polyurethane resins include(meth)acrylic resins, polyester resins, acrylonitrile-butadiene typecopolymers, polyamide resins, polyvinylbutyral, nitrocellulose,styrene-butadiene type copolymers, polyvinyl alcohol resins, acetalresins, epoxy type resins, phenoxy type resins, polyether resins,polyfunctional polyethers such as polycaprolactones, polyamide resins,polyimide resins, phenol resins, polybutadiene elastomers, chlorinatedrubbers, acrylic rubbers, isoprene rubbers, and epoxy-modified rubbers.

[0042] Examples of the thermosetting resins include phenol resins, epoxyresins, polyurethane resins, urea resins, butyral resins, formal resins,melamine resins, alkyd resins, silicone resins, acrylic reactive resins,polyamide resins, epoxy-polyamide resins, saturated polyester resins,and urea formaldehyde resins.

[0043] Preferably, a crosslinking agent is used to harden the binderresin. While various polyisocyanates, especially diisocyanates, aresuitably used as the crosslinking agent, at least one selected fromtolylene diisocyanate, hexamethylene diisocyanate, and methylenediisocyanate is particularly preferred. It is particularly preferredthat these crosslinkers are modified with a compound having a pluralityof hydroxyl groups, such as trimethylolpropane, or that they areprovided in the form of an isocyanulate-type crosslinker in which threemolecules of a diisocyanate compound have been bound. In this manner,the crosslinkers can bind to functional groups present in the binderresins to thereby crosslink the resin. Preferably, the crosslinkingagent is used in an amount of 10 to 30 parts by mass with respect to 100parts by mass of the binder resin. In general, such thermosetting resinscan be cured by heating them in an oven at 50 to 70° C. for 12 to 48hours.

[0044] Also, the above-described binder resins may be modified to beelectron-beam sensitive by introducing (meth)acrylic double bonds usingknown techniques. Several techniques for carrying out this modificationare known: urethane modification, in which an adduct of tolylenediisocyanate (TDI) and 2-hydroxyethyl(meth)acrylate (2-HEMA) is reactedwith the resin; modified urethane modification, in which a monomer (suchas 2-isocyanate ethyl(meth)acrylate) that includes one or more ethylenicunsaturated double bonds and one isocyanate group within one moleculebut not a urethane bond is used; and ester modification, in which acompound having a (meth)acryl group and an carboxylic anhydride or adicarboxylic acid is reacted with a resin having a hydroxyl group or acarboxylic acid group. Of these techniques, modified urethanemodification is preferred since, according to this technique, the resindoes not become brittle even when a high proportion of vinyl chloridetype resin is used and the technique provides coatings with highdispersibility and good surface smoothness.

[0045] When such an electron beam-curable binder resin is used, a knownpolyfunctional acrylate may be added in an amount of 1 to 50 parts bymass, preferably 5 to 40 parts by mass, with respect to 100 parts bymass of the binder resin, so as to enhance the crosslinking of theresin.

[0046] The amount of the binder resin used in the lower non-magneticlayer is preferably in the range of 10 to 100 parts by mass, and morepreferably 12 to 30 parts by mass, with respect to 100 parts by mass ofthe carbon black and the inorganic powders other than carbon blackcombined. Too small an amount of the binder may result in a decreasedproportion of the binder resin in the lower non-magnetic layer and,thus, insufficient coating strength. Conversely, too large an amount ofthe binder may lead to a dispersion failure upon preparation of thecoating for the lower non-magnetic layer. As a result, the desired flatnon-magnetic layer surface can no longer be obtained.

[0047] When necessary, the lower non-magnetic layer contains alubricant. The lubricant may be any of known lubricants, includingsaturated or unsaturated fatty acids, fatty acid esters, and sugars,which may be used either individually or as a mixture of two or more. Apreferred lubricant may comprise a mixture of two or more fatty acidswith different melting points or a mixture of two or more fatty acidesters with different melting points. Such a lubricant is advantageousin that it can be adapted to any temperature condition under which themagnetic recording medium is used. The lubricant is continuouslydelivered to the surface of the medium.

[0048] Specific examples of the fatty acids include straight-chainedsaturated fatty acids, such as stearic acid, palmitic acid, myristicacid, lauric acid, and erucic acid; branched saturated fatty acids, suchas isocetyl acid, and isostearic acid; and unsaturated fatty acids, suchas oleic acid, linoleic acid, and linolenic acid.

[0049] Examples of the fatty acid esters include straight-chainedsaturated fatty acid esters, such as butyl stearate, and butylpalmitate; branched saturated fatty acid esters, such as isocetylstearate, and isostearyl stearate; unsaturated fatty acid esters, suchas isostearyl oleate; fatty acid esters of unsaturated alcohols, such asoleyl stearate; esters formed of unsaturated fatty acids and unsaturatedalcohols, such as oleyl oleate; esters of diols, such as ethyleneglycoldistearate; esters formed of diols and unsaturated fatty acids, such asethyleneglycol monooleate, ethyleneglycol dioleate, and neopentylglycoldioleate; and esters formed of sugars and saturated or unsaturated fattyacids, such as sorbitan monostearate, sorbitan tristearate, sorbitanmonooleate, and sorbitan trioleate.

[0050] While the amount of the lubricant in the lower non-magnetic layercan be adjusted depending on its purpose, the lubricant is preferablyused in an amount of 1 to 20% by mass with respect to the total mass ofthe carbon black and the inorganic powders other than carbon black.

[0051] The coating for forming the lower non-magnetic layer is preparedby adding an organic solvent to the above-described components. Such anorganic solvent may be any organic solvent and is typically one or acombination of two or more solvents selected from various solvents,including ketone type solvents, such as methyl ethyl ketone, methylisobutyl ketone, and cyclohexanone; and aromatic type solvents, such astoluene. The organic solvent is preferably used in an amount of 100 to900 parts by mass with respect to 100 parts by mass of the carbon black,the various inorganic powders other than carbon black, and the binderresin combined.

[0052] The lower non-magnetic layer typically has a thickness of 0.1 to2.5 μm, and preferably 0.3 to 2.3 μm. The lower non-magnetic layer, whentoo thin, becomes more likely to be affected by the surface roughness ofthe non-magnetic support. As a result, the surface smoothness of thenon-magnetic layer is adversely affected, as is the surface smoothnessof the magnetic layer. This often results in a decreased electromagneticconversion characteristic. Also, too thin a non-magnetic layer leads toan increased light transmittance, causing problems when tape ends aredetected by the changes in the light transmittance. On the other hand,making a non-magnetic layer thicker than a certain thickness would notcorrespondingly improve the performance of the magnetic recordingmedium.

Upper Magnetic Layer

[0053] The upper magnetic layer contains at least a ferromagnetic powderand a binder resin

[0054] The average major axis length of the ferromagnetic powder ispreferably 0.1 μm or less. By using a ferromagnetic powder with a shortmajor axis length, the filling factor of the coating can be improved,making transfer from the back coat layer less likely. The average majoraxis length of the ferromagnetic powder preferably falls within a rangefrom 0.03 to 0.10 μm. If the average major axis length of theferromagnetic powder exceeds 0.1 μm, then increasing the filling factorof the coating becomes impossible, and the layer becomes prone totransfer from the back coat layer. In contrast if the average major axislength of the ferromagnetic powder is less than 0.03 μm, then themagnetic anisotropy weakens, making orientation more difficult, andincreasing the likelihood of a decrease in output.

[0055] A preferred ferromagnetic powder for use in the present inventionis a magnetic metal powder or a planar hexagonal fine powder. Themagnetic metal powder preferably has a coersive force Hc of 118.5 to 237kA/m (1500 to 3000 Oe), a saturation magnetization ss of 120 to 160Am²/kg (emu/g), an average major axis length of 0.03 to 0.1 μm, anaverage miner axis length of 10 to 20 nm, and an aspect ratio of 1.2 to20. Also, the magnetic recording medium made by using the magnetic metalpower preferably has an Hc value of 118.5 to 237 kA/m (1500 to 3000 Oe).The planar hexagonal fine powder preferably has a coersive force Hc of79 to 237 kA/m (1000 to 3000 Oe), a saturation magnetization ss of 50 to70 Am²/kg (emu/g), an average planar particle size of 30 to 80 nm, and aplate ratio of 3 to 7. Also, the magnetic recording medium made by usingthe planar hexagonal fine powder preferably has an Hc value of 94.8 to173.8 kA/m (1200 to 2200 Oe).

[0056] The average major axis length of the ferromagnetic powder can bedetermined by separating the ferromagnetic powder from a tape fragment,taking a photograph of a sample of the ferromagnetic powder using atransmission electron microscope (TEM), and measuring the lengths of themajor axes of the powder based on this photograph. One example of thisprocess is described below. (1) Remove the back coat layer from a tapefragment by wiping with a solvent. (2) Immerse the remaining tapefragment, which still comprises the lower non-magnetic layer and theupper magnetic layer on the non-magnetic support, in a 5% aqueoussolution of NaOH, and heat with constant stirring. (3) Isolate thecoatings that have separated from the non-magnetic support, wash withwater, and dry. (4) Subject the dried coatings to ultrasound treatmentin methyl ethyl ketone (MEK), and collect the magnetic powder byadsorption onto a magnetic stirrer. (5) Separate the magnetic powderfrom the residual liquid and dry. (6) Collect the magnetic powdersobtained in steps (4) and (5) using a special mesh, prepare a TEMsample, and photograph the sample using a TEM. (7) Measure the lengthsof the major axes of the magnetic powder from the photograph, andcalculate the average length (sample number: n=100).

[0057] The production of the magnetic metal powder uses a ferricoxyhydrate as a starting material. This material can be obtained byblowing an oxidative gas through an aqueous suspension of a ferrous saltand an alkali. A preferred ferric oxyhydrate is a-FeOOH. In a firstprocess for producing a-FeOOH, a ferrous salt is neutralized with analkali hydroxide to form an aqueous suspension of Fe(OH)₂, and anoxidative gas is then blown into the suspension to form a needle-shapeda-FeOOH product. In a second process for producing a-FeOOH, a ferroussalt is neutralized with an alkali carbonate to form an aqueoussuspension of FeCO₃, and an oxidative gas is then blown into thesuspension to form a spindle-shaped a-FeOOH product.

[0058] The ferrous salt for use in these processes may be any of ferrouschloride, ferrous nitride, and ferrous sulfate. The alkali hydroxide foruse in the first process may be potassium hydroxide, sodium hydroxide,or aqueous ammonia. The alkali carbonate for use in the second processmay be sodium carbonate, sodium bicarbonate, or ammonium carbonate.

[0059] In the first process, it is preferred to use 2 to 10 times asmuch alkali as required to neutralize the ferrous salt, so that theoxidation of Fe(OH)₂ takes place under highly basic conditions. Thea-FeOOH product obtained in this manner is suitable for making amagnetic metal powder that is fine, has no branches, and offers a highdispersibility and a high packing ability. It is necessary that theprocess be carried out under highly basic conditions to ensure that theresulting particles are unbranched. As it is known, one way to controlthe particle size is by controlling the reaction temperature and thevolume of the oxidative gas blown into the suspension. Alternatively,the particle size may be controlled by carrying out the neutralizationof the ferrous salt with the alkali in the presence of a metal salt,such as a salt of Ni, Co, Al, and Si, and then carrying out theoxidation.

[0060] The second process tends to produce fine particles of aspindle-shaped, unbranched a-FeOOH product that have uniform particlesizes. In the second process, the particle size can be controlled byvarying the ferrous concentration in the aqueous suspension, thereaction temperature, and the volume of the oxidative gas blown into thesuspension. As with the first process, the particle size can also becontrolled by the addition of Ni, Co, or the like.

[0061] One exemplary method for producing a magnetic metal powder willnow be described in which the needle-shaped a-FeOOH product obtained inthe first process is used as a starting material. First, ferrous salt isneutralized with twice as much or more of an alkaline hydroxide as isrequired to just neutralize the ferrous salt to form an alkalinesuspension of Fe(OH)₂. An oxidative gas is then blown into thesuspension to obtain a needle-shaped a-FeOOH product. To control theneedle-shaped ratio and the shape of the a-FeOOH product, the ferroussalt may be doped with metals such as Ni, Co, Zn, Cr, Mn, Zr, Al, Si, P,Ba, Ca, Mg, Cu, Sr, Ti, Mo, Ag, and rare earth elements. Theseheterogenous metals may be uniformly mixed with the ferrous salt or theymay be added during the reaction. The amounts added can be empiricallydetermined by the desired shape and the size of the product.

[0062] In this process, the ferrous salt is neutralized with an alkalito form a suspension of Fe(OH)₂, which is then oxidized to producea-FeOOH. By using twice as much or more of the alkaline as is requiredto neutralize the ferrous salt, the resultant a-FeOOH can be used as astarting material to produce a magnetic metal powder with high coersiveforce. While the degree of branching of the resulting a-FeOOH can befurther reduced by adding the alkali in larger excess, the alkali, whenadded in excessive amounts of 10 times or more, does not further improvethe effect and thus is not effective.

[0063] Also, it is necessary that the a-FeOOH particles required toproduce a favorable magnetic metal powder have a size such that itsspecific surface area as measured in BET value falls within the range of60 to 130 m²/g. The specific surface area that is less than 60 m²/gindicates that the particles are too large to provide a high coersiveforce and thus are not suitable as a magnetic material used in a singlewavelength region. In comparison, the specific surface area that isgreater than 130 m²/g indicates that the particles are too small toprovide a high coersive force although they may exhibitsuperparamagnetism. Too large a specific surface area also indicates awide distribution of coersive force, which may be due to non-uniformparticles.

[0064] Next, at least one of Ni, Co, Al, Si and rare earth elements isadded to the a-FeOOH, which may or may not be doped with Ni, Co, Zn, Cr,Mn, Zr, Al, Si, P, Ba, Ca, Mg, Cu, Sr, Ti, Mo, Ag, and a rare earthelement. The addition is typically carried out by neutralizing differentmetal salts with an acid or an alkali to deposit film of fine crystalsof hydroxides on the surface of the particles. While Ni, Co, and rareearth elements may not have to be deposited on the surface of thea-FeOOH particles, provided that the a-FeOOH product is doped withsufficient amounts of the dopants, these elements may be furtherdeposited on the surface of the particles when it is desired to increasethe amounts of the elements present in the a-FeOOH product since thea-FeOOH product can only be doped to a limited degree. The metalelements are preferably present in the magnetic metal powder in thefollowing ranges, where figures indicate the ratio by mass of each metalassuming the mass of iron to be 100:

[0065] Ni=0.3-8.0

[0066] Co=3.0-45.0

[0067] Al=0.5-8.0

[0068] Si=0.5-8.0 and

[0069] rare earth element=0.2-10.0,

[0070] provided that Al+Si=2.0-15.0.

[0071] The rare earth metal is at least one selected from the groupconsisting of La, Ce, Pr, Nd, Sm, Gd, Dy, and Y. The metals may beeffectively used in combinations. Preferably, the metals are added inthe form of water-soluble salts, such as chlorides, sulfates, andnitrates. Si is preferably added in the forms of sodium metasilicate,sodium orthosilicate, and water-glass. The metals are deposited in thefollowing order: First, Ni and Co, which form an alloy and serve tocontrol magnetic characteristics of the magnetic metal powder, aredeposited, followed by deposition of Al and Si, which serve to preventthe sintering of the particles by heat. The rare earth metals, which actto increase a force, can achieve the effect more significantly when theyare present internally, though they are somewhat effective whendeposited with Al and/or Si.

[0072] After predetermined amounts of the metals have been deposited,the metals are thoroughly washed with water, are dried, and are thenheat-treated at 300 to 800° C. in a non-reductive atomosphere. If theheat treatment is carried out at temperatures below 300° C., then theresulting a-Fe₂O₃ particles, generated in the dehydration of a-FeOOH,tend to contain numerous pores. As a result, the characteristics of thereduced magnetic metal powder are deteriorated. On the other hand, ifthe heat treatment is carried out at temperatures higher than 800° C.,then the a-Fe₂O₃ particles start to melt and are deformed or sintered.As a result, the characteristics of the resulting magnetic metal powderare deteriorated.

[0073] Following the heat treatment, the magnetic metal powder isreduced at a temperature of 300° C. to 600° C. in a stream of hydrogengas. This results in the formation of an oxide film on the surface ofthe particles and, as a result, the magnetic metal powder is obtained.To reduce the amounts of water-soluble sodium ions and water-solublecalcium ions in the magnetic metal powder, the purity of water for usein the above-described process may be increased, or a sodium- orcalcium-free alkali may be used.

[0074] The following processes are known for the production of hexagonalferrite. Any of these processes may be properly used.

[0075] (i) Glass crystallization technique, in which barium oxide, ironoxide, a metal oxide to replace iron, and boron oxide as a glass-formingmaterial are mixed to form a ferrite composition, which in turn ismelted and is then quenched to form an amorphous body. Subsequently, theamorphous body is again subjected to a heat treatment, is washed, and isthen crashed into a barium ferrite crystal powder.

[0076] (ii) Aqueous heat reaction technique, in which a solution ofmetal salts of barium ferrite composition is neutralized with an alkali,followed by removal of the by-products. The solution is then heated at100° C. or above and is subsequently washed, dried, and then crashedinto a barium ferrite crystal powder.

[0077] (iii) Co-precipitation technique, in which a solution of metalsalts of barium ferrite composition is neutralized with an alkali,followed by removal of the by-products. The solution is subsequentlydried, is treated at 1100° C. or below, and is then crashed into abarium ferrite crystal powder.

[0078] To reduce the amounts of water-soluble sodium ions andwater-soluble calcium ions in the hexagonal ferrite powder, the purityof water for use in each of the above-described processes (i), (ii), and(iii) may be increased, or a sodium- or calcium-free alkali may be used.

[0079] The ferromagnetic powder preferably contains the water-solublesodium ions in an amount of 70 ppm or less, more preferably 50 ppm orless. Also, the ferromagnetic powder preferably contains the watersoluble-calcium ion in an amount of 30 ppm or less, more preferably 20ppm or less. When contained in amounts greater than the specified range,these ions may form salts with organic acids (in particular, fattyacids) present in the coating. Such salts may seep out to the surface ofthe coating, causing drop-outs or an increase in the error rate.

[0080] Preferably, such a ferromagnetic powder is contained in an amountof about 70 to 90% by mass with respect to the magnetic layer. Too largean amount of the ferromagnetic powder leads to a decreased amount of thebinder and tends to result in a decreased surface smoothness aftercalendering. Conversely, too small an amount of the ferromagnetic powdercannot achieve high reproduction output.

[0081] The magnetic layer may contain any suitable binder, such as athermoplastic resin, thermosetting or thermoreactive resin, andradiation-curable (electron beam- or UV-curable) resin. These binderresins are used in proper combinations depending on the characteristicsof the magnetic recording medium and the conditions for the process. Thebinders can be properly selected from those described with reference tothe lower non-magnetic layer.

[0082] The amount of the binder resin used in the magnetic layer ispreferably in the range of 5 to 40 parts by mass, and more preferably 10to 30 parts by mass, with respect to 100 parts by mass of theferromagnetic powder. Too small an amount of the binder may result in adecreased strength and, thus, a decreased running durability of themagnetic layer. Conversely, too large an amount of the binder may leadto a decreased amount of the ferromagnetic powder, thus lowering theelectromagnetic conversion characteristics.

[0083] The magnetic layer further contains an abrasive having a Mohshardness of 6 or higher for the purposes of increasing the mechanicalstrength of the magnetic layer and preventing clogging of the magnetichead. Examples of the abrasives are those with a Mohs hardness of 6 orhigher, preferably 9 or higher, including a-alumina (Mohs hardness=9),chromium oxide (Mohs hardness=9), silicon carbide (Mohs hardness=9.5),silicon oxide (Mohs hardness=7), aluminum nitride (Mohs hardness=9), andboron nitride (Mohs hardness=9.5). Preferably, at least one of theseabrasives is added to the magnetic layer. The abrasives are generallyamorphous and thus serve to prevent clogging of the magnetic head and toincrease the coating strength.

[0084] The abrasive has an average particle size of for example 0.01 to0.2 μm, preferably 0.05 to 0.2 μm. If the average particle size of theabrasive is too large, then the projections from the surface of themagnetic layer become significant, causing a decrease in theelectromagnetic conversion characteristics, an increase in thedrop-outs, and an increase in the head wear. Conversely, if the averageparticle size of the abrasive is too small, then the protrusions fromthe surface of the magnetic layer will become relatively small, leadingto insufficient prevention of clogged heads.

[0085] In general, the average particle size is measured using atransmission electron microscope. The amount of the abrasive istypically in the range of 3 to 25 parts by mass, preferably in the rangeof 5 to 20 parts by mass, with respect to 100 parts by mass of theferromagnetic powder.

[0086] When necessary, the magnetic layer may further contain adispersing agent such as a surfactant, a lubricant such as a higherfatty acid, a fatty acid ester and, a silicone oil, and various otheradditives.

[0087] A preferred coating for forming the magnetic layer can beprepared by adding an organic solvent to the above-described components.Such an organic solvent may be any suitable organic solvent and may bethose used in the lower non-magnetic layer.

[0088] The magnetic layer typically has a thickness of 0.03 to 0.30 μm,preferably 0.10 to 0.25 μm. The magnetic layer that is too thick canresult in an increase in the self-demagnetization loss and the thicknessloss.

[0089] In the present invention, the smoothness of the surface of themagnetic layer is an important factor. The centerline average roughness(Ra) of the surface of the magnetic layer is preferably within a rangefrom 1.0 to 8.0 nm, and even more preferably from 2.0 to 7.0 nm. At Ravalues less than 1.0 nm, the surface becomes overly smooth, causing adeterioration in the running stability and an increase in the likelihoodof trouble during running of the tape. In contrast, if the Ra valueexceeds 8.0 nm, then the surface of the magnetic layer becomes overlyrough, resulting in a deterioration in the reproduction output and otherelectromagnetic conversion characteristics in reproducing systems thatuse MR heads.

[0090] The ten-point average centerline roughness (Rz) of the surface ofthe magnetic layer is preferably within a range from 5 to 25 nm, andeven more preferably from 5 to 20 nm. At Rz values less than 5 nm, thesurface becomes overly smooth, causing a deterioration in the runningstability and an increase in the likelihood of trouble during running ofthe tape. In contrast, if the Rz value exceeds 25 nm, then the surfaceof the magnetic layer becomes overly rough, resulting in a deteriorationin the reproduction output and other electromagnetic conversioncharacteristics in reproducing systems that use MR heads.

[0091] As increasingly shorter recording wavelengths are being used incurrent high-density recording systems, it is desirable to take intoconsideration not only the aforementioned surface roughness values (Raand Rz) of the magnetic layer, but also the surface roughness ofmicroscopic areas (for example, areas of approximately 10 μm×10 μm) inorder to more accurately evaluate the output, the error rate and othercharacteristics of the magnetic layer. In the case of a recording andreproducing system with a minimum recording wavelength of 0.6 μm orshorter, in terms of the surface roughness determined solely from themicroscopic areas, the magnetic layer must have a surface roughness,reported as an AFM surface roughness (Ra) value, of 6.0 nm or less, andpreferred values fall within a range from 2.0 to 6.0 nm, and even morepreferably within a range from 2.0 to 5.0 nm. If the AFM surfaceroughness value Ra exceeds 6.0 nm, then the spacing increases, causingthe likelihood of an increase in the error rate. In contrast, if the AFMsurface roughness value Ra is less than 2.0 nm, then the scratch andabrasion resistance of the magnetic layer deteriorates, which can causea decrease in the running reliability.

[0092] The AFM surface roughness Ra value of the surface of the magneticlayer is determined based on Ra as defined in JIS-B-0601, which isdetermined from a surface roughness curve obtained on the basis ofmeasurements taken using an atomic force microscope. More specifically,a probe with a radius of curvature of 10 nm or less, and preferablywithin a range from 2 to 10 nm, is used to measure an area of 10 μm×10μm, and image processing is then performed to determine the centerlineaverage surface roughness Ra.

[0093] In the present invention, the number of concavities with a depthof 30 nm or greater in the surface of the magnetic layer must be 5 per 1cm² of surface area or less. Concavities with a depth of 30 nm orgreater cause spacing loss, and cause the likelihood of an increase inthe error rate. If the number of these concavities per 1 cm² of surfacearea is 6 or greater, then the error rate increases. There is no minimumrestriction on the number of these concavities, and the fewer thebetter. The examples described below display values of approximately 0.1concavities/cm².

[0094] The number of the above concavities is determined using anoptical interference type three-dimensional roughness meter, bymeasuring the concavities with a diameter of 10 to 60 μm and a depth of30 nm or greater, adjusting the interference intensity for an opticalmicroscope (50 to 100× magnification), counting the number of the aboveconcavities, for example counting at least 3 fields of view for a ½ inchwide tape of length from 1 to 5 cm, and then calculating the arithmeticmean of the counted values.

Back Coat Layer

[0095] The back coat layer serves both to ensure the running stabilityof the magnetic recording medium and to prevent the magnetic layer frombeing electrified. This layer contains carbon black, a non-magneticinorganic powder other than carbon black, and a binder resin.

[0096] The back coat layer preferably contains 30 to 80% by mass carbonblack with respect to the amount of the back coat layer. If the amountof carbon black is too small, then the electrification preventing effectof the back coat layer may be reduced, as may the running stability. Inaddition, the light transmittance of the magnetic medium may beincreased, which may pose problems in systems in which tape ends aredetected by the changes in the light transmittance. On the other hand,if the amount of carbon black is excessively large, then the strength ofthe back coat layer will be reduced, resulting in a decrease in therunning durability. Carbon black may be of any type that is commonly inuse and preferably has a particle size in the range of about 5 to 500nm. The particle size of carbon black is generally measured by atransmission electron microscope.

[0097] It is preferred that the carbon black contain minimal amounts ofwater-soluble sodium ions and water-soluble calcium ions: the amount ofthe water-soluble sodium ions is preferably 500 ppm or less, morepreferably 300 ppm or less while the amount of the water-soluble calciumions is preferably 300 ppm or less, more preferably 200 ppm or less.When contained in amounts greater than the specified range, thewater-soluble sodium ions or the water-soluble calcium ions may formsalts with organic acids (in particular, fatty acids) present in thecoating. Such salts may seep out to the surface of the coating, causingdrop-outs or an increase in the error rate.

[0098] Aside from carbon black, the back coat layer may further containvarious non-magnetic inorganic powders to control the mechanicalstrength of the magnetic recording medium. Examples of such inorganicpowders include a-Fe₂O₃, CaCO₃, titanium oxide, barium sulfate, anda-Al₂O₃. The amount of the non-magnetic inorganic powder is preferablyin the range of 0.1 to 20 parts by mass, and more preferably in therange of 0.5 to 15 parts by mass, with respect to 100 parts by mass ofcarbon black. The non-magnetic inorganic powder preferably has anaverage particle size of 0.01 to 0.5 μm. Too small an amount of thenon-magnetic inorganic powder may lead to insufficient mechanicalstrength of the back coat layer, whereas too large an amount of thepowder may result in substantial abrasion of guide members that slideagainst the tape or may cause scratches on the magnetic layer.

[0099] Aside from the above-described materials, the back coat layer maycontain binders such as thermoplastic resins, thermosetting orthermoreactive resins, and radiation-curable (electron beam- orUV-curable) resins. These binder resins are used in proper combinationsdepending on the characteristics of the magnetic recording medium andthe conditions for the process. The binders can be properly selectedfrom those described with reference to the lower non-magnetic layer.

[0100] The amount of the binder resin for use in the back coat layer ispreferably in the range of 15 to 200 parts by mass, and more preferablyin the range of 50 to 180 parts by mass, with respect to 100 parts bymass of carbon black and the non-magnetic inorganic powder combined. Ifthe amount of the binder resin is too large, then the friction betweenthe tape and the guide rollers and other components that the tape slidesagainst will become excessive, resulting in decreased running stabilityand making the tape prone to running failures. Too large an amount ofthe binder resin can also cause problems such as the back coat layer'sblocking to the magnetic layer. Conversely, if the amount of the binderresin is too small, then the strength of the back coat layer will bedecreased, often resulting in reduced running durability.

[0101] When necessary, a dispersing agent such as a surfactant, alubricant such as a higher fatty acid, a fatty acid ester, and asilicone oil, and various other additives may be added to the back coatlayer.

[0102] Such a lubricant is properly selected from those described withreference to the lower non-magnetic layer. While the amount of thelubricant in the back coat layer may be adjusted depending on itspurpose, the lubricant is preferably contained in an amount of 1 to 20%by mass with respect to the total mass of carbon black and the inorganicpowders other than carbon black.

[0103] The coating for forming the back coat layer is prepared by addingan organic solvent to the above-described components. Such an organicsolvent may be any organic solvent and is preferably selected from thosedescribed with reference to the lower non-magnetic layer. The organicsolvent is preferably used in an amount of 100 to 900 parts by mass withrespect to 100 parts by mass of the carbon black, the various inorganicpowders other than carbon black, and the binder resin combined.

[0104] After calendering, the back coat layer typically has a thicknessof 1.0 μm or less, preferably from 0.1 to 1.0 μm, and more preferablyfrom 0.2 to 0.8 μm. If the back coat layer is too thick, then thefriction between the back coat layer and a guide roller and othercomponents that the tape slides against becomes excessive, resulting ina decreased running stability. On the other hand, the back coat layer,when too thin, becomes susceptible to abrasion during the running of themagnetic recording medium. Also, when the back coat layer is too thin,the surface smoothness of the back coat layer is reduced due to thesurface roughness of the non-magnetic support. As a result, when theback coat layer is hardened by heat, the surface roughness of the backcoat layer tends to be transferred to the surface of the magnetic layerto cause a reduction in each of the power output at higher range, S/N,and C/N.

Non-Magnetic Support

[0105] The non-magnetic support may be formed from any suitable materialselected from various flexible materials and rigid materials dependingon its purposes and may be sized and shaped into a desired size andshape, such as tape-like shape, depending on the standard that thesupport is required to meet. For example, a preferred flexible materialmay be a polyester, such as polyethylene terephthalate, or polyethylenenaphthalate; a polyolefin, such as polypropylene; or various otherresins, such as polyamide, polyimide, and polycarbonate.

[0106] Preferably, the non-magnetic support is 3.0 to 15.0 μm thick andmay be shaped into any desired shape, such as tape-, sheet-, card-, ordick-like shape. The non-magnetic support can be made from variousmaterials selected to suit its shape and requirements.

[0107] The non-magnetic support for use in the present inventiontypically has a surface roughness as measured in the centerline averagesurface roughness Ra of 20 nm or less, preferably 15 nm or less. Ifnecessary, the surface roughness of the non-magnetic support can beadjusted as desired depending on the size and the amount of the filleradded to the non-magnetic support. Examples of the fillers includeoxides and carbonates of Ca, Si, Ti, and Al, and fine powders of organicresins such as acryl-based resins. Particularly preferred arecombinations of Al₂O₃ and organic resin fine powders.

Production Process

[0108] A magnetic recording medium according to the present inventionwith the formation described above can be produced by conducting a stepA of forming the lower non-magnetic layer by applying a non-magneticlayer coating onto one surface of the non-magnetic support andsubsequently drying and curing the coating, a step B of forming theupper magnetic layer, after the step A, by applying a magnetic layercoating onto the cured lower non-magnetic layer and subsequently dryingthe coating, and a step C of forming the back coat layer by applying aback coat layer coating onto the other surface of the non-magneticsupport and subsequently drying the coating, and performing acalendering step D following completion of both the step A and the stepC.

[0109] Each of the processes for producing the coatings for the backcoat layer, the lower non-magnetic layer, and the magnetic layerinvolves at least a kneading step and a dispersing step, and otheroptional steps that are carried out before or after each of the firsttwo steps, including a mixing step, a viscosity-adjusting step, and afiltration step. Each step may consist of two or more sub-steps. Any ofthe materials for use in the present invention, including theferromagnetic powder, the non-magnetic inorganic powder, the binder, theabrasive, the carbon black, the lubricant, and the solvent, may be addedat the beginning of, or during, any of the aforementioned steps. Eachmaterial may be added in two or more separate steps.

[0110] For kneading/dispersing of each coating, known productiontechniques can be used during part of, or throughout, the step. For thekneading step, however, it is preferred to use a high-power kneader suchas a continuous kneader or a pressure kneader. A continuous kneader or apressure kneader is used to knead/mix the ferromagnetic powder or thenon-magnetic inorganic powder, the binder, and a small amount of thesolvent. The slurry is preferably kneaded at a temperature of 50° C. to110° C.

[0111] A dispersion medium having a high specific gravity is preferablyused in each coating. Preferred examples include ceramic medium such aszirconia and titania. Conventional glass beads are undesirable sinceupon dispersing, the beads wear to produce water-soluble sodium ions andwater-soluble calcium ions as impurities of the coating.

[0112] The coating method includes various known methods such as agravure coating, a reverse roll coating, a die nozzle coating, or barcoating methods.

[0113] When producing the magnetic recording medium, the lowernon-magnetic layer formation step A, the upper magnetic layer formationstep B, the back coat layer formation step C, and the calendering step Dcan be performed in a number of different sequences. The importantfactors to ensure are that the upper magnetic layer is formed aftercuring of the lower non-magnetic layer, and that the calendering step Dis not performed with either of the surfaces of the non-magnetic supportexposed, but is rather performed following the formation of a layer oneach of the surfaces of the non-magnetic support.

[0114] Possible sequences in which the steps can be performed includethe sequences shown below.

[0115] Step A→Step B→Step C→Step D

[0116] Step A→Step C→Step D→Step B→Step D

[0117] Step A→Step C→Step B→Step D

[0118] Step C→Step A→Step D→Step B→Step D

[0119] Step C→Step A→Step B→Step D

[0120] Alternatively, the application of the back coat layer could alsobe performed simultaneously with the application of the lowernon-magnetic layer or the upper magnetic layer.

[0121] As described above, in the case where the calendering step D isperformed following completion of both the step A and the step C butprior to the step B, an additional calendering step D is preferablyperformed following completion of step B in order to smooth the surfaceof the magnetic layer. Multiple repetitions of the calendering step Dmay also be performed within other step sequences.

[0122] A preferred calender roll system uses a combination of metalrolls and heat resistant plastic, elastic rolls such as epoxy,polyester, nylon, polyimide, polyamide, or polyimideamide plastic rolls(carbon, metals, or other inorganic compounds may be blended with theheat-resistant plastics). Furthermore, treatment with combinations ofmetal rolls is preferred as it produces a smoother magnetic layersurface. In order to ensure a smoother surface, the metal rolls arepositioned so as to contact the magnetic layer surface. Plastic elasticrolls are typically positioned on the other side, so as to contact theback coat layer, although metal rolls are preferred.

[0123] The calendering treatment temperature is preferably 70° C. ormore, and even more preferably within a range from 90° C. to 110° C. Thelinear pressure is preferably 1.9×10⁵ N/m (200 kg/cm) or more, and evenmore preferably within a range from 2.4×10⁵ N/m (250 kg/cm) to 3.8×10⁵N/m (400 kg/cm), and the process speed is typically within a range from20 m/min to 900 m/min.

[0124] In the present invention, because the upper magnetic layer isapplied following curing of the lower non-magnetic layer (a so-calledwet-on-dry application, W/D), the types of problems that are seen inwet-on-wet applications (W/W), in which the magnetic layer is appliedwhile the non-magnetic layer is still wet, such as disturbance of theinterface between the non-magnetic layer and the magnetic layer, anddeterioration in the surface smoothness due to surface swelling, whichcauses an increase in the error rate, do not arise.

[0125] In the present invention, because the calendering step D is notperformed with either of the surfaces of the non-magnetic supportexposed, but is rather performed following the formation of a layer oneach of the surfaces of the non-magnetic support, there is no directcontact between the non-magnetic support base and the calender rolls,meaning scraping of the base or the fillers contained within the basedoes not occur. As a result, the calendering can be performed extremelyeffectively. If the non-magnetic support base and the calender rollscome in direct contact, then the presence of scraped filler and the likegenerated by this contact increases the likelihood of concavitiesdeveloping in the lower non-magnetic layer and the upper magnetic layer,making it impossible to achieve a medium with excellent surfacesmoothness as required by the present invention. Concavities generatedin the lower non-magnetic layer or the upper magnetic layer as a resultof filler scrapings or the like are particularly large, with a diameterof 10 to 60 μm, although are usually shallow, with a depth of 30 to 100nm. However, these concavities have a marked effect on the error rate inrecording and reproducing systems in which the minimum recordingwavelength is 0.6 μm or shorter.

[0126] Following completion of the aforementioned step A, step B, stepC, and step D, the heat curing treatment is conducted, thereby curingthe upper magnetic layer and the back coat layer. The magnetic recordingmedium in the present invention can be preferably obtained by performingadditional calendaring on both of the upper magnetic layer surface andback coat layer surface after the heat curing treatment.

EXAMPLES

[0127] The present invention will now be described in detail withreference to examples, which are not intended to limit the scope of theinvention in any way.

Example 1 Preparation of a Coating for Lower Non-Magnetic Layer

[0128] (Preparation of Binder Solution) Electron beam-curable vinylchloride 45 parts by mass type resin NV30 wt % (vinyl chloride-epoxy-containing monomer copolymer, average degree of polymerization = 310,epoxy content = 3 wt %, S content = 0.6 wt %, acryl content = 6 units/ 1molecule, Tg = 60° C.) Electron beam-curable polyester polyurethane 16parts by mass resin NV40 wt % (polar group —OSO₃ Na- containingpolyester polyurethane, number average molecular weight = 26000) Methylethyl ketone (MEK)  2 parts by mass Toluene  2 parts by massCyclohexanone  2 parts by mass

[0129] The composition above is placed in a hyper mixer and was stirredto form a binder solution.

[0130] (Kneading)

[0131] The following composition was placed in a pressure kneader andwas kneaded for 2 hours. Needle-shaped a-Fe₂O₃ 85 parts by mass (TODAKOGYO, DB-65, average major axis length = 0.11 μm, BET(specific surfacearea) = 53 m²/g) Carbon black 15 parts by mass (MITSUBISHI CHEMICAL Co.,Ltd., #850B, average particle size = 16 nm, BET = 200 m²/g, DPB oilabsorbance = 70 ml/100 g) a-Al₂O₃  5 parts by mass (SUMITOMO CHEMICALCo., Ltd., HIT-60A, average particle size = 0.20 μm) o-phthalic acid  2parts by mass Binder solution 67 parts by mass

[0132] To the slurry resulting after the kneading process, the followingcomposition was added to optimize the viscosity of the slurry for thedispersing process. MEK 40 parts by mass Toluene 40 parts by massCyclohexanone 40 parts by mass

[0133] (Dispersing)

[0134] The resulting slurry was subjected to a dispersing process in ahorizontal pin mill filled to 75% capacity with zirconia beads (TORAY,torayceram φ 0.8 mm).

[0135] (Viscosity-Adjusting Solution)

[0136] The following composition was placed in a hyper mixer and wasstirred to form a viscosity-adjusting solution. Stearic acid  1 part bymass Butyl stearate  1 part by mass MEK 30 parts by mass Toluene 30parts by mass Cyclohexanone 30 parts by mass

[0137] (Viscosity Adjustment and Final Coating)

[0138] To the slurry resulting after the dispersing process, thesolution prepared above was added, and the mixture was mixed/stirred andwas again subjected to the dispersing process in a horizontal pin mill,filled to 75% capacity with zirconia beads (TORAY, torayceram φ 0.8 mm),to obtain a coating. The coating was circulated for filtration through adepth filter with an absolute filtration accuracy of 1.0 μm. This gave afinal coating product for the lower non-magnetic layer.

Preparation of a Coating for Magnetic Layer

[0139] (Preparation of Binder Solution) Vinyl chloride type resin (ZEON11 parts by mass Corporation, MR-110) Polyester polyurethane resin NV30%17 parts by mass (TOYOBO, UR-8300) MEK  7 parts by mass Toluene  7 partsby mass Cyclohexanone  7 parts by mass

[0140] The composition above was placed in a hyper mixer and wasmixed/stirred to form a binder solution.

[0141] (Kneading)

[0142] The following composition was placed in a pressure kneader andwas kneaded for 2 hours. a-Fe magnetic powder 100 parts by mass (Hc =1885Oe, Co/Fe = 20 at %, ss = 138 emu/g, BET = 58 m²/g, average majoraxis length = 0.10 μm) a-Al₂O₃  6 parts by mass (SUMITOMO CHEMICAL Co.,Ltd., HIT-60A, average particle size = 0.20 μm) a-Al₂O₃  6 parts by mass(SUMITOMO CHEMICAL Co., Ltd., HIT-82, average particle size = 0.13 μm)Phosphoric ester  2 parts by mass (TOHO CHEMICAL, PHOSPHANOL RE610)Binder solution  49 parts by mass

[0143] To the slurry resulting after the kneading process, the followingcomposition was added to optimize the viscosity of the slurry for thedispersing process. MEK 100 parts by mass Toluene 100 parts by massCyclohexanone  75 parts by mass

[0144] (Dispersing)

[0145] The resulting slurry was subjected to a dispersing process in ahorizontal pin mill filled to 75% capacity with zirconia beads (TORAY,torayceram φ 0.8 mm).

[0146] (Viscosity-Adjusting Solution)

[0147] The following composition was placed in a hyper mixer and wasmixed/stirred for 1 hour to form a viscosity-adjusting solution. Stearicacid  1 part by mass Butyl stearate  1 part by mass MEK 100 parts bymass Toluene 100 parts by mass Cyclohexanone 250 parts by mass

[0148] (Viscosity Adjustment)

[0149] To the slurry resulting after the dispersing process, thesolution prepared above was added, and the mixture was mixed/stirred andwas again subjected to the dispersing process in a horizontal pin mill,filled to 75% capacity with zirconia beads (TORAY, torayceram φ0.8 mm),to obtain a coating. The coating was circulated for filtration through adepth filter with an absolute filtration accuracy of 1.0 μm.

[0150] (Final Coating)

[0151] To 100 parts by mass of the coating resulting after filtration,0.82 part by mass of an isocyanate compound (NIPPON POLYURETHANEINDUSTRY Co., Ltd., Coronate L) were added. The mixture wasmixed/stirred and was then circulated for filtration through a depthfilter with an absolute filtration accuracy of 1.0 μm to obtain a finalcoating product for the magnetic layer.

Preparation of a Coating for Back Coat Layer

[0152] (Preparation of Binder Solution) Nitrocellulose  50 parts by mass(ASAHI KASEI, BTH1/2) Polyester polyurethane resin NV30% 110 parts bymass (TOYOBO, UR-8300) MEK 200 parts by mass Toluene 200 parts by massCyclohexanone 200 parts by mass

[0153] The composition above was placed in a hyper mixer and wasmixed/stirred to form a binder solution.

[0154] (Dispersing)

[0155] The following composition was placed in a ball mill and wasprocessed for 24 hours to thoroughly disperse the components. Carbonblack  75 parts by mass (CABOT Co., Ltd., BLACK PEARLS 800, averageparticle size = 17 nm, BET = 220 m²/g) Carbon black  10 parts by mass(CABOT Co., Ltd., BLACK PEARLS 130, average particle size = 75 nm, BET =25 m²/g) BaSO₄  15 parts by mass (SAKAI CHEMICAL INDUSTRY Co., Ltd.,BF-20, average particle size = 30 nm) Copper oleate  5 parts by massCopper phthalocyanine  5 parts by mass a-almina  1 part by mass (TAIMEICHEMICALS Co., Ltd., TM-DR, average particle size = 0.23 μm) Bindersolution 760 parts by mass

[0156] (Viscosity-Adjusting Solution)

[0157] The following composition was placed in a hyper mixer and wasstirred to form a viscosity-adjusting solution. MEK 220 parts by massToluene 220 parts by mass Cyclohexanone 220 parts by mass

[0158] (Viscosity Adjustment)

[0159] To the slurry resulting after the dispersing process, thesolution prepared above was added, and the mixture was mixed/stirred andwas again subjected to the dispersing process for 3 hours in a ballmill. The resultant coating was circulated for filtration through adepth filter with an absolute filtration accuracy of 3.0 μm.

[0160] (Final Coating)

[0161] To 100 parts by mass of the coating resulting after filtration,1.1 parts by mass of an isocyanate compound (NIPPON POLYURETHANEINDUSTRY Co., Ltd., Coronate L) was added. The mixture was mixed/stirredand was then circulated for filtration through a depth filter with anabsolute filtration accuracy of 3.0 μm to obtain a coating for the backcoat layer.

Production of Magnetic Recording Tape

[0162] The above-prepared coating for the lower non-magnetic layer wasapplied to one surface of a 6.1 μm thick polyethylene terephthalate filmat a line speed of 100 m/min to a dry thickness of 2.0 μm. The film wasthen dried in an oven into which a 100° C. hot air stream was sent at aspeed of 15 m/sec. Subsequently, the film was irradiated with anelectron beam at a dose of 4.5 Mrad and was then wound.

[0163] The above-prepared coating for the magnetic layer was thenapplied over the cured lower non-magnetic layer at a line speed of 100m/min to a dry thickness of 0.20 μm. While still wet, the coating wasexposed to a magnetic field generated by a solenoid (5000 Oe) to orientthe magnetic powder and was dried in an oven into which a 100° C. hotair stream was sent at a speed of 15 m/sec. Subsequently, theabove-prepared coating for the back coat layer was applied to the othersurface of the polyethylene terephthalate film to a dry thickness of 0.6μm. The film was then dried in an oven into which a 100° C. hot airstream was sent at a speed of 15 m/sec and was then wound. In thismanner, an uncalendered magnetic tape web was obtained.

[0164] Subsequently, the uncalendered magnetic tape web was fed outthrough feed rollers, and calendering of both the magnetic layer surfaceand the back coat layer surface was performed using a calenderingapparatus with the roll configuration described below, under conditionsincluding a temperature of 100° C., a linear pressure of 350 kg/cm, anda process speed of 100 m/min. The treated tape web was then wound.

[0165] Roll Configuration:

[0166] 10 nips comprising combinations of a metal roll (S) and a metalroll (S).

[0167] The wound roll was placed in an oven for 24 hours at 60° C. toeffect heat curing. Following heat curing, the tape web was subjected toa second calendering treatment using the same roll configuration and thesame conditions described above, and was then rewound. The tape was thenslit into ½ inch wide (12.65 mm) strips to obtain a magnetic tape.

Example 2

[0168] A magnetic tape was produced in the same manner as in Example 1,with the exception of subjecting the uncalendered magnetic tape web totwo calendering treatments using a calendering apparatus with the rollconfiguration described below.

[0169] Roll Configuration:

[0170] 10 nips comprising combinations of a metal roll (S) and apolyamide resin roll (R).

[0171] The metal rolls were positioned so as to contact the magneticlayer surface, and the elastic rolls were positioned so as to contactthe back coat layer surface.

Example 3

[0172] A magnetic tape was produced in the same manner as in Example 2,by subjecting the uncalendered magnetic tape web to two calenderingtreatments using the same calendering apparatus as in Example 2, butwith the exception of altering the conditions to include a temperatureof 100° C., a linear pressure of 300 kg/cm, and a process speed of 100m/min.

Example 4

[0173] A magnetic tape was produced in the same manner as in Example 2,by subjecting the uncalendered magnetic tape web to two calenderingtreatments using the same calendering apparatus as in Example 2, butwith the exception of altering the conditions to include a temperatureof 80° C., a linear pressure of 300 kg/cm, and a process speed of 100m/min.

[0174] In each of the comparative examples described below, the samecoatings as those described in Example 1 were used for the coatings forthe lower non-magnetic layer, the upper magnetic layer, and the backcoat layer respectively.

Comparative Example 1

[0175] The above-prepared coating for the lower non-magnetic layer wasapplied to one surface of a 6.1 μm thick polyethylene terephthalate filmat a line speed of 100 m/min to a dry thickness of 2.0 μm. The film wasthen dried in an oven into which a 100° C. hot air stream was sent at aspeed of 15 m/sec. Subsequently, the film was subjected to calenderingtreatment using a calendering apparatus with the same roll configurationas in Example 1 (as described below), under conditions including atemperature of 100° C., a linear pressure of 350 kg/cm, and a processspeed of 100 m/min. The film was then irradiated with an electron beamat a dose of 4.5 Mrad, and the tape web was subsequently wound.

[0176] Roll Configuration:

[0177] 10 nips comprising combinations of a metal roll (S) and a metalroll (S).

[0178] The above-prepared coating for the magnetic layer was thenapplied over the cured lower non-magnetic layer at a line speed of 100m/min to a dry thickness of 0.20 μm. While still wet, the coating wasexposed to a magnetic field generated by a solenoid of 5000 Oe to orientthe magnetic powder, and was then dried in an oven into which a 100° C.hot air stream was sent at a speed of 15 m/sec. The resulting tape webwas then rewound.

[0179] Subsequently, the above-prepared coating for the back coat layerwas applied to the other surface of the polyethylene terephthalate filmto a dry thickness of 0.6 μm. The film was then dried in an oven intowhich a 100° C. hot air stream was sent at a speed of 15 m/sec. The tapeweb was subsequently subjected to a second calendering treatment usingthe same roll configuration and the same conditions described above, andwas then rewound.

[0180] The wound roll was placed in an oven for 24 hours at 60° C. toheat cure the tape. Following heat curing, the raw tape was subjected toyet another calendering treatment using the same roll configuration andthe same conditions described above, and was then rewound. The tape wasthen slit into ½ inch wide (12.65 mm) strips to obtain a magnetic tape.

Comparative Example 2

[0181] A magnetic tape was produced in the same manner as in ComparativeExample 1, with the exception of subjecting the magnetic tape web tothree calendering treatments using a calendering apparatus with the rollconfiguration described below.

[0182] Roll Configuration:

[0183] 10 nips comprising combinations of a metal roll (S) and apolyamide resin roll (R).

[0184] The metal rolls were positioned so as to contact the magneticlayer surface, and the elastic rolls were positioned so as to contactthe back coat layer surface.

Comparative Example 3

[0185] A magnetic tape was produced in the same manner as in ComparativeExample 2, by subjecting the magnetic tape web to three calenderingtreatments using the same calendering apparatus as Comparative Example2, but with the exception of altering the conditions to include atemperature of 100° C., a linear pressure of 300 kg/cm, and a processspeed of 100 m/min.

Comparative Example 4 No Lower Non-Magnetic Layer

[0186] The above-prepared coating for the magnetic layer was applied toone surface of a 6.1 μm thick polyethylene terephthalate film at a linespeed of 100 m/min to a dry thickness of 0.20 μm. While still wet, thecoating was exposed to a magnetic field generated by a solenoid of 5000Oe to orient the magnetic powder, and was then dried in an oven intowhich a 100° C. hot air stream was sent at a speed of 15 m/sec. The filmwas then irradiated with an electron beam at a dose of 4.5 Mrad, and theresulting tape web was wound.

[0187] Subsequently, the above-prepared coating for the back coat layerwas applied to the other surface of the polyethylene terephthalate filmto a dry thickness of 0.6 μm. This film was dried in an oven into whicha 100° C. hot air stream was sent at a speed of 15 m/sec, and the tapeweb was then rewound. This process completed the formation of theuncalendered magnetic tape web.

[0188] Subsequently, the uncalendered magnetic tape web was fed outthrough feed rollers, and calendering of both the magnetic layer surfaceand the back coat layer surface was performed using a calenderingapparatus with the same roll configuration as in Comparative Example 2(as described below), under conditions including a temperature of 100°C., a linear pressure of 300 kg/cm, and a process speed of 100 m/min.The treated tape web was then wound.

[0189] Roll Configuration:

[0190] 10 nips comprising combinations of a metal roll (S) and apolyamide resin roll (R).

[0191] The metal rolls were positioned so as to contact the magneticlayer surface, and the elastic rolls were positioned so as to contactthe back coat layer surface.

[0192] The wound roll was placed in an oven for 24 hours at 60° C. toeffect heat curing. Following heat curing, the tape web was subjected toa second calendering treatment using the same roll configuration and thesame conditions described above, and was then rewound. The tape was thenslit into ½ inch wide (12.65 mm) strips to obtain a magnetic tape.

Evaluation of Magnetic Tapes

[0193] (Centerline Average Surface Roughness Ra, Ten-Point AverageSurface Roughness Rz)

[0194] Using a Talystep system (manufactured by Taylor Hobson), thecenterline average surface roughness Ra (nm) and the ten-point averagesurface roughness Rz (nm) were determined for the surface of themagnetic layer based on measurements taken according to JIS-B-0601. Theconditions for the measuring instrument included a filter condition of0.30 to 9.0 Hz, a probe of 0.1×2.5 μm stylus, probe pressure of 2 mg,measurement speed of 0.03 mm/sec, and a measured length of 500 μm.

[0195] (AFM Centerline Average Surface Roughness: Ra(nm))

[0196] Using AutoProbe M5 atomic force microscope (AFM) (ThermoMicroscpes), the AFM average surface roughness Ra was determined.

[0197] The number of times that the analysis was performed: N=3

[0198] Probe: Silicon single crystal probe (Nanosensors, radius ofcurvature=10 nm)

[0199] Scan mode: non-contact mode

[0200] Scan area: 10 μm×10 μm

[0201] Pixel number: 512×512 data points

[0202] Scan rate: 0.6 Hz

[0203] Measurement environment: at room temperature in the atmosphere

[0204] Data processing: Secondary slope correction was performed alonghorizontal and vertical directions for the whole image data.

[0205] (Number of Concavities with a Depth of 30 nm or Greater)

[0206] The number of concavities with a depth of 30 nm or greater per 1cm² of surface area of the magnetic layer was determined in thefollowing manner. A 12.65 mm wide tape was cut into 3 cm lengths toprepare three tape fragment samples. The surface of the magnetic layerof each tape fragment sample was inspected under an optical microscopeto detect concavities. The depth of the detected concavities wasmeasured using an optical interference type three-dimensional roughnessmeter (manufactured by WYKO), with a cutoff of 0.25 nm and a measurementrange of 250 μm×250 μm. The number of concavities with a depth of 30 nmor greater in the surface of the magnetic layer was then counted. Thesame procedure was repeated for the remaining two tape fragment samples,and the number of concavities with a depth of 30 nm or greater wascounted for each sample. The arithmetic mean of the three concavitycount values was then determined. Using this arithmetic mean, the numberof concavities per 1 cm² of surface area was calculated.

[0207] (Error Rate)

[0208] To determine the error rate, data were written using a MIG head(head width: 24 μm) on all tracks throughout the length of the tape andwere subsequently read out using an MR head (head width: 14 μm). Theminimum recording wavelength was 0.37 μm and the number of tracks was450.

[0209] The results of the above analyses are shown in Table 1. As isevident from Table 1, each of the magnetic tapes of Examples 1 through 4displayed an AFM surface roughness Ra for the surface of the magneticlayer of 6 nm or less, and in each case the number of concavities with adepth of 30 nm or greater in the surface of the magnetic layer wasrestricted to 5 concavities per 1 cm² of surface area or less, and theerror rate was extremely low.

[0210] In Comparative Examples 1 to 3, because calendering was alsoconducted following formation of the lower non-magnetic layer but priorto the formation of the back coat layer, there was direct contactbetween the base film and the calender rolls, resulting in a markedincrease in the number of concavities when compared with thecorresponding Examples 1 to 3. This increases in the number ofconcavities caused an increase in the error rate. TABLE 1 ApplicationCalendering method for Step Roll Ra Rz AFM Ra Concavity Write errormulti-layer sequence* configuration Condition (nm) (nm) (nm)(number/cm²) (error/MB) Example 1 W/D A→B→C→D S-S 100° C., 2.8 17.1 4.50.1 0.148 350 kg/cm Example 2 W/D A→B→C→D S-R 100° C., 2.8 18.3 4.3 0.30.170 350 kg/cm Example 3 W/D A→B→C→D S-R 100° C., 3.1 20.9 4.8 0.60.226 300 kg/cm Example 4 W/D A→B→C→D S-R  80° C., 3.1 21.7 5.8 4.80.500 300 kg/cm Comparative W/D A→D→B→C→D S-S 100° C., 3.1 19.4 4.3 16.70.780 Example 1 350 kg/cm Comparative W/D A→D→B→C→D S-R 100° C., 3.221.0 5.3 23.3 1.286 Example 2 350 kg/cm Comparative W/D A→D→B→C→D S-R100° C., 3.6 23.8 5.8 26.0 1.539 Example 3 300 kg/cm Comparative Singlelayer B→C→D S-R 100° C., 5.6 35.0 7.1 0.5 1.800 Example 4 300 kg/cm

What is claimed is:
 1. A magnetic recording medium, comprising a lowernon-magnetic layer containing at least a non-magnetic powder and abinder resin on one surface of a non-magnetic support, an upper magneticlayer containing at least a ferromagnetic powder and a binder resin onthe lower non-magnetic layer, and a back coat layer on the other surfaceof the non-magnetic support, wherein the thickness of the upper magneticlayer is within the range from 0.03 to 0.30 μm, the AFM surfaceroughness Ra of the upper magnetic layer is 6 nm or less, and the numberof concavities with a depth of 30 nm or greater in the surface of theupper magnetic layer is 5 per 1 cm² of surface area or less.
 2. Themagnetic recording medium according to claim 1, wherein the averagemajor axis length of the ferromagnetic powder is 0.1 μm or less.
 3. Themagnetic recording medium according to claim 1 or 2, wherein the mediumis used in a recording and reproducing system in which the minimumrecording wavelength is 0.6 μm or shorter.